A driving method of a liquid crystal apparatus using chiral nematic liquid crystal is disclosed in Japanese Examined Patent Publication No. 1-51,818 (corresponding to U.S. Pat. No. 4,239,345). The patent specification describes an initial orientational condition in the initial state under non-application of voltage, two metastable states, and a method for switching over between the two metastable states. However, the patent specification contains however no description about a practicable method for driving, and further, it discloses nothing about a driving method of a matrix display which is at present the most practical liquid crystal display.
Under these circumstances, the present inventors filed applications for Japanese Unexamined Patent Publication No. 6-230,751 and Japanese Unexamined Patent Publication No. 7-175,041 relating to driving methods for matrix display and achieved a practicable liquid crystal display unit and a driving method thereof. More specifically, the present inventors prepared a liquid crystal apparatus formed by holding a chiral nematic liquid crystal having an initial twist angle .PHI. (for example, 180.degree.) between a pair of substrates. A stripe-shaped electrode is formed on each of the substrates. The conventional driving method is as follows.
A giant pulse sufficient to transfer the liquid crystal molecules to homeotropic state is applied to the liquid crystal layer held between the pair of substrates. Then, after a certain interval of a delay time, a selection pulse using a critical value as the reference is applied to the liquid crystal to create a 0.degree. uniform state (.PHI.-180.degree.) or a 360.degree. (.PHI.+180.degree.) twisted state after relaxation of the homeotropic state. A display is achieved by the foregoing .PHI.-180.degree. state and the .PHI.+180.degree. state: the former is referred to as ON state, and the latter, as OFF state. This driving method is based on the pulse response of the liquid crystal.
FIG. 7 illustrates an example of a driving waveform representing another conventional driving method. (a) of FIG. 7 is a common waveform applied to a scanning electrode, and (b) of FIG. 7 is an example of a data waveform applied to a signal electrode. The common waveform is applied to the above-mentioned scanning electrode during a prescribed period of time comprising a reset period 8, a delay period 9, a selection period 10, and a non-selection period 11.
More specifically, a giant pulse is applied during the reset period, and an interval is placed during the delay period. During the K-th line selection period 10 of matrix display, a selection pulse having an amplitude of selecting an ON-state or an OFF-state of the display is applied. During the remaining non-selection period, the other scanning electrode is selected. This conventional driving method is based on line-sequential scanning.
The giant pulse applied during the reset period 8 is a pulse having a pulse height of at least 17 V, and requires a sustaining time of about 1 to 2 ms. The selection pulse applied during the selection period 10 should preferably have a voltage three to four times as high as the data voltage applied to the signal electrode.
The delay period 9 should be a time of several hundred .mu.s, and voltage should be null (reference voltage Vc) during the delay period and the non-selection period 11.
The data waveform (b) should show a symmetrical form of amplitude on the positive and negative sides in relation to the reference voltage Vc. When the phase of data waveform is the same as the phase of selection pulse, the display OFF-state is selected, and when it is an antiphase, the display ON-state is selected. Except for the reset period 8, therefore, the process would be based on the same method of general passive matrix addressing.
Inversion of signal for AC conversion is conducted for every interval of several times of the selection period (1H) (nH; n is a positive integer) within one frame, and a DC component is canceled by reversing the waveforms of the immediately preceding frame. The waveform, not shown here, applied to the liquid crystal media is equal to the difference between the common signal and the data signal. No problem is therefore caused if a waveform having the same level of voltage to that in this example is applied. Another example could be put into practice by dividing the voltage levels of the common signals and the data signals into two groups of low and high voltages and selecting certain voltage levels of these signals between these two groups in a hopping manner. Examples of these practices are described in the foregoing Japanese Unexamined Patent Publication.
In a liquid crystal apparatus using a super twisted nematic liquid crystal (STN liquid crystal), on the other hand, rapid attenuation of the display state not experienced in the conventional cumulative response, i.e., attenuation of transmissivity becomes larger, when the response time of liquid crystal materials are shorter, and the resultant lower contrast phenomenon is known as a frame response.
As a measure to solve this problem, a concept in which a plurality of scanning electrodes are simultaneously addressed (or scanning lines) is developed. (This is hereinafter referred to as the "multi-line driving method", and abbreviated as the "MLS driving method"). These circumstances are described in detail in the Japanese Unexamined Patent Publication No. 5-100,642 and Japanese Unexamined Patent Publication No. 4-148,845.According to these documents, in the aforesaid driving method, a plurality of selection periods are provided in a single scanning waveform, and are dispersed in a frame. In this driving method, therefore, a necessary transmissivity is determined and an ON/OFF state of display is obtained by accumulating responses of liquid crystal for individual selecting period. This driving method is utilizing cumulative response of the liquid crystal and the root mean square (RMS) response effect.
FIG. 8 shows an example of the conventional driving method: the driving waveforms for simultaneously selecting of four scanning electrodes. Common waveforms R1 to R4 applied to the four scanning electrodes are as shown in FIG. 8. That is, the selection periods S1 to S4 are dispersed within a frame, and a selection pulse is applied to the liquid crystal equally at four every period t. A property known as orthogonal normality as referred to in the aforesaid patent application is imparted between the individual common waveforms. More specifically, the selection pulse applied to the individual selection periods (S1 to S4) of the four scanning electrodes R1 to R4, by assuming 1 for the positive side and 0 for the negative side relative to a reference voltage (Vc), is expressed by a determinant. A selection voltage is set so that this determinant satisfies orthogonality.
Data waveforms C1 and C2 are shown in FIG. 8, in which examples of data signals to each four rows accessed simultaneously are illustrated. Voltage of the data signal is set at any of five voltage levels in total relative to the reference potential (Vc: i.e., zero). Specifically, a data signal is determined in response to the four selected rows and a display state of the column crossing these rows (there are 2.sup.4 =16 ON/OFF combinations).
Applying to a practical circuit, a common signal and data signals are passed through 4 exclusive OR gates, and the level of a voltage to be applied to LCD will be fixed by counting the output states of the gates.
Thus, the voltage to be applied to the liquid crystal is as effective as a RMS voltage which is the difference between the common signals and the data signals in a frame period. Therefore, a display state in compliance with the RMS voltage is available even by a driving method using a selection period divided into four. AC conversion of the driving waveform is accomplished through inversion for every frame. AC conversion of a voltage applied to the liquid crystal layer is achieved through two frames.
By using the driving method shown in FIG. 7, the present inventors could drive a conventional liquid crystal apparatus at a duty ratio of 1/240, and succeed in driving such a large-capacity liquid crystal apparatus. In order to achieve a liquid crystal display of a larger capacity by improving a driving method, it was necessary to reduce the selection period for the writing pulse and achieve a faster response time of a liquid crystal, however this requirement inevitably led to a narrower driving voltage margin of the display element for the existing liquid crystal materials.